1) Field of the Invention
This invention relates to an easy-opening closure for hermetic sealing of an open end of a retortable container and an easy-opening container that is hermetically sealed by such a closure.
2) Description of Related Art
A variety of closures are known for the hermetic sealing of a container, such as conventional tin-plated steel cans that are widely used for containing food products. Retortable containers are those that can withstand a pasteurization or retort process comprising heat and pressure for preserving the food contents of the container. During retort, the container can be subjected to temperatures above 212° F. and up to 250° F. under pressures of 15 to 30 psi.
Easy-opening containers are those that can be opened without undue effort and without the use of a special tool such as a rotary can opener. In order for an easy-opening container to be retortable, the closure must be sufficiently strong to resist stresses that develop as a result of the retort heat and pressure but easily overcome during opening. One conventional easy-opening, retortable container includes a closure that is stronger in shear than tension. The closure is strong enough to withstand the shear force that develops during retort, while a relatively small tensile force is required to open the container. For example, U.S. Pat. No. 5,752,614, titled “Easy-Opening Closure for Hermetic Sealing a Retortable Container,” to Nelson describes an easy-opening closure. The closure includes a metal end ring that is adapted to be seamed to an open end of a retortable container and defines a central opening of the container. An edge of the end ring that defines the central opening is preferably rolled. A membrane patch covers the opening and is bonded to the end ring. The bond is unaffected during retort processing, but has a predetermined tensile force strength that is preferably less than 5 psi to allow peeling of the membrane patch from the end ring. Thus, the container can be retorted and subsequently easily opened. However, because the rolled edge is positioned within the container, the contents of the container can contact the edge. Contaminants trapped within the rolled edge, for example, debris or moisture trapped during manufacture of the end ring, can be introduced into the container and thereby contaminate the contents. Additionally, corrosion of the edge can result, for example during retort, also resulting in contamination of the contents of the container. In some embodiments, a membrane ring, which extends from the end ring to the membrane patch, acts as a barrier between the edge and the contents of the container. During retort, however, gas and/or moisture contained in the rolled edge can expand and stress the bonds that hold the membrane ring to the end ring and the membrane patch. A failure of either bond can result in contamination of the contents, which can be difficult to detect without opening the container.
Another container closure known in the art includes a metal end ring R with a folded edge E, as shown in FIG. 2. The edge E is folded outward from the container, i.e., away from the contents of the container. Further, a membrane M is affixed to the ring R by a bond B such that the edge E is hermetically sealed from the contents of the container. By preventing contact between the edge E and the contents, the risk of corrosion of the edge and contamination of the contents is reduced, but contact between the edge E and the contents may occur after the closure is opened. Additionally, upon removing the membrane M, the folded edge E is exposed to the user, thereby detracting from the aesthetic appeal of the closure. Further, if the single bond B is strong enough to resist the pressures associated with retort, the bond may be difficult for the user to overcome to open the container.
Thus, there exists a need for an easy-opening closure for hermetically sealing an open end of a retortable container and an easy-opening container that is hermetically sealed by such a closure. The closure should be strong enough to withstand the stresses induced during retort, but easily removed by a user. Additionally, the closure should reduce the likelihood of contamination to the contents of the container, for example, during assembly of the container, during retort, and after opening the container.